Direct Ink Writing Printability – Ashby-like Plots for Guided Design

Abstract

Direct Ink Writing (DIW), an extrusion-based 3D printing technique, offers a broad application space. As such, the technique continues to find use in biomedical, flexible electronic, ceramic, and energy device applications, among others. With this broad application space comes an expanding material library of inks with diverse rheological and microstructural properties. This begs the question: what constitutes a printable ink? How does one define printability? And how does one design for printability? Researchers currently have a broad understanding of what constitutes a printable ink. However, time and time again, inks with unique rheological properties and formulations are printed. Currently, ink synthesis and ink characterization for DIW take a linear analysis approach – one in which rheological and material specifications are an afterthought. To progress DIW while understanding the fundamental questions discussed above, it may be necessary to take a design approach. The design approach melds understanding of the yield stress fluid microstructure and resultant rheological properties, providing a wholistic view of the DIW fluid parameter space. Implementing design enables one to target rheological properties for a given fluid microstructure or implement new yield stress microstructures with predictable relevant rheological parameters. This ability ultimately accelerates DIW material implementation, reduces experimental time, and opens the door for novel microstructure exploration and ink development. In this work, we explore DIW printability through a series of cases studies which involve attractive glass and repulsion dominated yield stress fluids. Through the development of a DIW rheological database, we establish the utility of Ashby-like plots in transitioning DIW to a design-based engineering approach. Through the Ashby plots and the clustering of yield stress fluid microstructures, we investigate and identify why defining the concept of printability remains elusive. Ultimately, we propose microstructurally-dependent targeted rheological parameters and demonstrate the utility of design for DIW to accelerate DIW ink implementation.